If a cylinder liner is worn, combustion gas leaks at an increased rate, and deteriorates the lubricant oil; and the resulting poor lubrication promotes the wear of the cylinder liner. This is a vicious cycle which brings about the wear of the engine as a whole, and a reduction in its efficiency. The cylinder liner is usually worn more heavily in the vicinity of the top dead center position of the piston than in any other area, since in the vicinity of top dead center, the higher pressure of the combustion chamber acts on the piston ring, and causes it to contact the cylinder liner very tightly. Moreover, as only a minimum of lubricant oil is supplied in the vicinity of top dead center, the oil film is broken, which causes higher friction between the piston ring and the cylinder liner, and the heat produced by such friction causes a fusion of metal between the piston ring and the cylinder liner. This phenomenon is called scuffing, and often occurs during the initial period of engine operation. A number of proposals have hitherto been made to prevent the wear of the cylinder liner in the vicinity of the top dead center position, but none of them have been successful in providing a satisfactory cylinder liner for a high-speed and high-output engine.
The noted proposals have intended to improve the scuffing resistance of the cylinder liner by forming a high hardness layer on its inner surface by chromium plating, spray coating or selective quenching. Chromium plating is, however, ineffective against corrosive fuel, such as high lead gasoline or low quality diesel oil. A spray coated layer is low in bonding strength, and likely to crack and peel off in the vicinity of the top dead center position due to high impact stress and thermal shock. Since a cylinder liner is heavily strained and deformed if hardened as a whole, it has been proposed that it be hardened only in the vicinity of top dead center, or ports (in the case of a two cycle engine), as shown, for example, in Japanese Laid-Open Patent Specification No. 32408/76. Although the hardening of a cylinder liner certainly improves its resistance to ordinary wear, it does not greatly improve its scuffing resistance, and hardly improves its corrosion resistance over an unhardened cylinder liner. Although it is proposed in, for example, Japanese Laid-Open Patent Specification No. 137362/78 that the scuffing resistance of a cylinder liner may be improved by hardening in a special pattern, the proposed treatment is complicated. Since a cast iron cylinder liner has a base of martensite structure in which graphite and carbide are dispersed, the hardness of the hardened layer decreases with a rise in temperature, though it is high at ordinary room temperature. If a certain temperature is reached, there occurs a fushion of metal, i.e., scuffing, between the metallic base structure of the cylinder liner and the piston ring.